I have reserved the color purple to use for this exact purpose. Any photo I want to mark for quick return, I label purple. I then just create a temporary filter for purple label and all of them appear. Since the filter bar works on what is currently in the filmstrip, this will even work when you have separate folders or even separate collections of photos.

how would it help in workflow?: E.G.: I currently have project to to go through all my images and re-edit and generally completely rethink how I want to present and curate them [about 100,000 images] So when going through a folder of thousands of images, curating them to put into new collection set with newly created collections, and creating new collections within it on the fly, checking IN those collections to see whether you have such and such a version in there, checking through snapshots of it etc wtrying a B&W version, back to big folder, remember that another image not in current view would go well, go find it, come back to same place in big folder, scroll through images , find another the belongs in a collection, , but decide to rename the collection (update) , checking in that collection whether the other images will fit the new name properly, look in QC to see if there are others in there that it reminded you of , the phone rings, your mother's carers says the electricity has gone off, get off the phone, try to remember back to big folder and the image position two images ago, to continue re-editing images along the way, (because you're getting better at it and suddenly see how something can be improved, or see another use for it, } , get fuddled where you were in the big folder, was it scroll up or down to that last image before you accidentally clicked on a another image in one of the collections that shifted your position again in the big folder and wonder whether you have a memory at all now you're over 60, pull your hair out wishing for bookmarks, because if the position of the over-riding task at hand was bookmarked you could shoot immediately back to that spot. ONE bookmarks would in fact suffice (but two or three may have uses for some). In fact ideally you could tell the bookmark to remember the folder and auto remember the last selection before you went of and did a handful of other things in other fielders before coming back to that exact last image in the right folder

Point is there are many many things during LR use that will not only take you away from the 'big' folder placement (i.e. the exact place in the one you want to get back to) but which will reset your position in that folder.

Every single day I wish for a bookmark/bookmarks, it is so frustrating and time wasting trying to get back to the point of curation you were initially working on.e.g. here I have just edited in: photoshop and taken it back into ghtroom and added it to a collection and now want to go back to where I was previously in my main folder with thousands of images, but I can't because in between I clicked on other collections and lost my place- If I had bookmarked it [or it was set to auto bookmark] I could just click the icon and return - hey presto!!